Sometimes in the progress of forefront research, one gets two answers
for the price of one. Based on X-ray structures recently determined at
ORNL, it now appears that phenolic extractants known for four decades
and recently discovered calixarene-crown extractants share features that
confer extremely high selectivity for cesium. Both types of extractants
have been examined intensively for use in nuclear-waste cleanup, but it
had not been understood that they possess the same ability to bond to
alkali metal cations through the pi-cloud of a benzene ring. In the case
of calixarene-crowns, the pi-bonding takes place in a preorganized molecular
cavity, whereas simple 2-benzylphenols apparently self-assemble about
the cation. With a better structural understanding of these compounds,
the ORNL Chemical Separations Group aided by calculations performed by
Ben Hay at PNNL is designing new modifications that may lead to yet higher
selectivity. If you'd like to learn more about this work, please have
a look at some of our recent papers
such as: J. C. Bryan, L. H. Delmau, B. P. Hay, J. B. Nicholas, L. M. Rogers,
R. D. Rogers, and B. A. Moyer, "Cesium Recognition by Supramolecular Assemblies
of 2-Benzylphenols with 2-Benzyl-phenolate." Struct. Chem.10,
187-203 (1999) and R. A. Sachleben, A. Urvoas, J. C. Bryan, T. J. Haverlock,
B. P. Hay, and B. A. Moyer, "Dideoxygenated Calix[4]arene Crown-6 Ethers
Exhibit Enhanced Selectivity for Cesium over Potassium and Rubidium,"
J. Chem. Soc., Chem. Commun. 1751-1752 (1999).

This research was sponsored by the Division
of Chemical Sciences, Office of Basic Energy Sciences, U. S. Department
of Energy, under contract number DE-AC05-96OR22464 with Oak Ridge National
Laboratory, managed by Lockheed Martin Energy Research Corp.